Electrochemical sensors are frequently used in occupational safety, medical engineering, process measuring engineering, environmental analysis, etc. Such sensors have electrode arrays with two, three or more electrodes, which are called auxiliary electrode. reference electrode, and working electrode(s). The reference electrode is eliminated in the two-electrode sensor, and the auxiliary electrode is called the counterelectrode. A so-called potentiostat is needed to operate these sensors. This potentiostat regulates the potential difference between the reference electrode and the working electrode(s) to a preset value. The measured signal, which depends on the substance concentration to be measured, is derived from the current of the working electrode(s). Thus, the potentiostat also has the function of measuring the current in these working electrodes.
An electrochemical sensor of the type mentioned in the introduction has been known from DE 197 24 888 A1, in which an electrochemical gas sensor is described, which has a sensor electrode array, which is connected to an operating electronic unit, which is integrated on a chip, and the operating electronic unit communicates with a separate microprocessor which is remote from the sensor proper. A potentiostat circuit, which includes an analog control circuit, is also accommodated on the chip in this prior-art sensor. A typical analog potentiostat circuit is shown in FIG. 3 of the present application. Such potentiostat circuits tend in some sensors to hunt or do not regulate the potential difference between the working electrode and the reference electrode with an optimal time response. Furthermore, sensor aging effects may cause the regulation to become unstable. This is due to the fact that the control parameters are determined by the dimensioning of the electric circuit components, as they are shown, e.g., in FIG. 3. Therefore, changes can be made in the fixed controller parameters defined by such circuits only by making hardware changes on the electronic circuit components, which implies practically such a considerable effort that such changes are not practical.
A general problem of many electrochemical sensors is that the highly sensitive electrode arrays often deliver only very weak signals with measured currents on the order of magnitude of a few nanoA. The problem of very weak electrical sensor signals will become even more acute in the future due to the low limit values specified in many areas of application, to which a sensor is to respond with a high level of reliability, because the limit values of the variables to be measured will be lowered rather than raised in many areas due to strict specifications. The evaluation of electrical sensor signals with measured currents in the nanoA range requires highly sensitive electric circuits for processing and further processing, which are inherently also susceptible to the effect of electromagnetic disturbances because of their high sensitivity. This problem is tackled in the sensor of the type mentioned in the introduction by integrating all circuits of the operating electronic unit on one chip, so that only digital signals must be transmitted from the chip to the microprocessor and vice versa, the digital communication with the microprocessor having a low fault liability. Nevertheless, the measured signal that is taken off from the potentiostat circuit is an analog signal, which is subsequently passed on to the next circuits of the operating electronic unit for further processing, and is susceptible to fault at this first stage.
An electronic memory (an electronically erasable programable read only memory—EEPROM) is frequently integrated in more recent sensors in order to store sensor data, such as the type of the sensor, potentials, calibration coefficients, etc. and to make them available to the operating electronic unit.
To monitor a larger area, e.g., on the premises of a plant, a plurality of measuring heads with a sensor each are placed in a distributed pattern. An operating electronic unit of its own, which is accommodated in the measuring head together with the sensor electrode array, is thus, of course, also needed for each sensor. The transmission of the measured values and the power supply to the measuring heads are carried out by means of a star-shaped network to and from a central point. A plurality of measuring heads must be installed and operated in such systems, which implies a great installation and maintenance effort in light of the fact that, as was described above, sensor aging effects or other changes may occur in individual sensors and require corresponding corrections.